1. Field of the Invention
The present invention relates to a dynamic air pressure bearing in which grooves for generating air pressure are disposed on either of a rotor or a stator.
2. Description of the Related Art
In general, a dynamic air pressure bearing is used to stably rotate a rotor not in contact with a stator.
There is known a dynamic air pressure bearing having a rotating member composed of either of a sleeve and a shaft which are fitted to each other. In this case, the sleeve and shaft are formed of an iron-base material and are subjected to heat-treatment or surface treatment for enhancing the life of the bearing; or they are formed of a ceramic material.
FIG. 6 is a schematic view of explaining an optical deflector; and FIG. 7 is a perspective view of a shaft 20 and a sleeve 30 constituting a dynamic air pressure bearing.
Referring to FIG. 6, an optical deflector is generally designated by reference numeral 1. In this optical deflector 1, the shaft 20 is erected and fixed at the center portion of a housing 10 as a base, and a cover 12 is mounted outside the housing 10 by way of a housing adaptor 11.
A stator core fixing stud 42 is mounted on the housing 10 by means of a screw 13, and a stator core 43 is fixed on the stator core fixing stud 42. A circuit board fixing stud 41, which is supported by the stator core 43, supports a circuit board 46 by means of a screw 15. A motor drive control circuit and the like are mounted on the circuit board 46, and the circuit board 46 is connected to a voltage supply unit (not shown) by way of a harness 48 and a connector 49.
Grooves 22 for generating a dynamic air pressure are formed around the outer peripheral portion of the stationary shaft 20, and the sleeve 30 is rotatably disposed outside the stationary shaft 20 with a gap G1. A magnet yoke 40 is inserted around the outer peripheral side of the lower portion of the rotating sleeve 30. A magnet 45, which is composed of an inner magnet 45a and an outer magnet 45b surrounding the stator core 43, is mounted inside the magnet yoke 40.
A rotary polygonal mirror 62 is supported over the magnet yoke 40 by means of a mirror flange 60. Reflection mirror surfaces 64 are formed around the outer periphery of the rotary polygonal mirror 62. A center screw 50 is attached to the top of the rotating sleeve 30, and a mirror cap 70 is mounted using a screw portion 52 of the center screw 50. The mirror cap 70 is so constructed as to press the rotary polygonal mirror 60 by way of a rotary polygonal mirror pressing force adjusting member 76, and which is intended to prevent the generation of the strain of the rotary polygonal mirror.
In the mirror cap 70, an air sump A1 is formed between the upper end surface of the sleeve 30 and the upper end surface of the stationary shaft 20. Fine holes 72, a balance correcting groove 73, a mirror cap fastening groove 74 and the like are provided at suitable portions of the mirror cap 70.
A damper 18 is mounted at the position opposed to the lower end surface of the sleeve 30 over the housing 10 for receiving the sleeve 30 upon stoppage of a motor. A magnetic detecting element 47 is provided on the circuit board 46, and a lens 80 is provided on the cover 12.
The operation of the optical deflector will be described below.
The magnet 45 (inner magnet 45a, outer magnet 45b) constituting the so-called scanner motor portion for rotating the rotary polygonal mirror 62 is composed of a permanent magnet, which generates a magnetic attracting force with the opposed stator core 43. The attraction force acts such that the opposed positions of the magnet 45 and the stator core 43 are prevented from being shifted in the axial direction (thrust direction) of the stationary shaft 20 of the motor.
Assuming that the fixed side of the stationary shaft 20 is directed downward and the opening side thereof is directed upward, when the magnet 45 is moved upward, there emerges a downward attracting component in the magnetic attraction force, and thereby the magnet 45 is returned downward. When the magnet 45 is moved downward, there emerges an upward attracting component, and thereby the magnet 45 is returned upward. The magnet 45 and the stator core 43 are thus opposed to each other at the specified axial position by the magnetic attraction force. In other words, the magnet 45 and the stator core 43 constitutes a magnet thrust bearing.
The magnetic detecting element 47 may include a Hall element. This detects a leakage flux of the magnet 45, that is, detects the passing of either the N-pole or the S-pole during the rotation of the magnet 45. The detection signal of the magnetic detecting element 47 is supplied to a control circuit section (not shown) by way of a wiring printed on the rotating board 46. On the basis of the detection signal, the control circuit section determines the direction of a current flowing a magnetic coil wound around each portion of the stator core 43. As a result, there is generated a force in the direction of continuing the rotation according to the correlation with the magnet 45. The magnet pole of the inner magnet 45a is set to be the same as that of the outer magnet 45b opposed to the inner magnet 45a.
When the rotating sleeve 30 is rotated, there is generated an air layer with a high pressure around the stationary shaft 20 (at the bearing gap G1). The pressure allows the rotating sleeve 30 to be floated from the stationary shaft 20, thus constituting a dynamic air pressure bearing.
In the figure, the grooves 22 for generating dynamic pressure are provided around the outer peripheral portion of the stationary shaft 20; however, they may be provided around the inner wall of the rotating sleeve 30 instead.
The air layer of the above gap G1 acts to keep constant the rotational center of the rotor portion. For example, when the rotating sleeve 30 is shifted in the right in the figure, the gap G1 in the right is made larger and the pressure in this gap is made smaller than that in the previous state. On the other hand, the gap in the left is made smaller, so that the pressure in this gap is made larger than that in the previous state. As a large or small change in the pressure becomes the above state, the rotating sleeve 30 is pushed in the left, and finally returned to the original position.
The cover, which serves to prevent dust as described above, is fixed on the housing adaptor by a screw. The cover is formed of plastic for reducing the cost. Moreover, to reduce the cost, the cover is often not used.
An incident light beam such as a laser beam is reflected by the reflection mirror surface 64 of the rotary polygonal mirror 62, and is emitted to a medium to be scanned such as a photosensible drum.
When the light beam is made incident to the reflection surface 64 and the rotary polygonal mirror 62 is rotated, the reflected light beam is gradually changed in the direction, and is thus deflected.
When the rotation proceeds and the next reflection mirror surface emerges, a light beam is made incident to this reflection mirror surface. Even at the reflection mirror surface, the light beam is deflected in the same manner as in the previous reflection mirror surface. Accordingly, the reflected light beams scan in a specified angular range, and the scanning rate thereof is dependent on the rotating speed of the polygonal mirror 62.
The dynamic air pressure bearing is of a stationary shaft type in which the shaft 20 is fixed; however, there is known a rotating shaft type in which a sleeve is fixed to a housing and a rotating shaft is inserted therein.
In addition, the dynamic air pressure bearing of this type has been disclosed, for example in Unexamined Japanese Patent Publication No. HEI 3-199714.
The prior art described above, however, has the following disadvantage: namely, since the bearing is constituted of the sleeve and shaft made of an iron-base material, it is poor in wear resistance, thereby tending to generate the melt-down phenomenon that the sleeve and shaft seize each other and obstructed from being rotated during the rotation of a motor. Namely, the prior art bearing is lack of the reliability as the motor. Moreover, there is known a ceramic made bearing for enhancing the wear resistance; however, it is difficult to be processed because of the high hardness of the ceramic, resulting in the increased cost, which is inconvenient in terms of mass-production.